Response to Infection

Host Defense Mechanisms

The body has several lines of defence against pathogens, including the skin, mucous membranes, and immune system.
The innate immune response involves non-specific defences such as physical barriers, chemical secretions and inflammation.
The adaptive immune response is more specific, involving the production of antibodies by B cells and the activity of cytotoxic T cells.

Cells Involved in Immune Response

Neutrophils are the most common type of white blood cell and are often the first on the scene of an infection, where they engulf and destroy pathogens.
Macrophages not only consume pathogens but also present antigens on their surfaces, activating other immune cells.
B cells produce antibodies specific to the detected antigen.
T cells consist of helper T cells, which stimulate B cells to produce antibodies, and cytotoxic T cells, which kill infected cells.

Antibodies and Antigen-Antibody Reactions

Antibodies, or immunoglobulins, are proteins produced by B cells that recognise and bind to antigens.
There are five major types of immunoglobulins: IgA, IgD, IgE, IgG, and IgM.
Each type has a specific role in the immune response, with IgG being the most abundant and involved in long-term immunity.
The bonding of antibodies to antigens facilitates phagocytosis, neutralises toxins and triggers the complement system, resulting in pathogen destruction.

Immune Response Phases

The primary immune response is the initial reaction to an antigen, featuring a lag phase while B and T cells are activated.
The secondary immune response is stronger and quicker due to memory cells produced in the primary response, providing long-term immunity.
Vaccines simulate a primary response, enabling quicker response upon actual exposure to the pathogen.

Immune System Pathologies

An autoimmune disease is when the immune system mistakenly attacks the body’s own cells, as happens in conditions like type 1 diabetes and rheumatoid arthritis.
Allergies are an overreaction of the immune system to harmless substances.
Immunodeficiency diseases, like HIV/AIDS, impair the immune system’s ability to fight infections.

Understanding these defensive processes will aid your grasp of many biological phenomena, from the effectiveness of vaccines to the basis of allergies and auto-immune diseases.